The Fully-Lit tool enables you to indicate surfaces in the image that are receiving full illumination. Marking an area with the Fully-Lit tool indicates that area should remain the same color and intensity in the reflectance map as in the original image.
In general, use Fully-Lit scribbles sparingly and with small strokes. Their main purpose is to set the intensity and color of a surface in the reflectance map. Often, a single stroke can have a global effect on the brightness and color of the reflectance map.
Image (A) shows a scene of a building and sky, where there is a significant shadow on the left side of the building. A single Bright/Dark Pair properly identifies the color of the two sides of the building, but the sky becomes saturated in the initial reflectance map (B).
Image (C) shows the original image with two Fully-Lit scribbles on the sky, and one on the lit side of the building. These scribbles fix the brightness of the sky, while maintaining the color of the building in the reflectance map (D).
Images (E) and (F) show the final reflectance map and the original image, respectively, for comparison.